Postweaning multisystemic wasting syndrome (PMWS) is a recently recognized disease of young pigs. The PMWS syndrome detected in Canada, the United States and France is clinically characterized by a gradual loss of weight and by manifestations such as tachypnea, dyspnea and jaundice. From the pathological point of view, it is manifested by lymphocytic or granulomatous infiltrations, lymphadenopathies and, more rarely, by hepatitis and lymphocytic or granulomatous nephritis (Clark E. G. (1997) Proc. Am. Assoc. Swine Prac. 499-501; La Semaine Veterinaire No. 26, supplement to La Semaine Veterinaire 1996 (834); La Semaine Veterinaire 1997 (857): 54; Nayar et al. (1997) Can. Vet. J. 38:385-387). Treatment and prevention of this disease are not currently available. Several lines of evidence, however, point to porcine circovirus as the etiologic agent of PMWS (Ellis et al. (1998) Can. Vet. J. 39:44-51). Circoviruses have been recovered from pigs with PMWS, and antibodies to porcine circovirus have been demonstrated in pigs with the disease.
A family of viruses, named Circoviridae, found in a range of plant and animal species and commonly referred to as circoviruses, are characterized as round, non-enveloped virions with mean diameters from 17 to 23.5 nm containing circular, single-stranded deoxyribonucleic acid (ssDNA). The ssDNA genome of the circoviruses represent the smallest viral DNA replicons known. As disclosed in WO 99/45956, at least six viruses have been identified as members of the family according to The Sixth Report of the International Committee for the Taxonomy of Viruses (Lukert et al. 1995, The Circoviridae, pp. 166-168. In Murphy, et al. (eds.) Virus Taxonomy, Sixth Report of the International Committee on Taxonomy of Viruses, Arch. Virol. 10 Suppl.).
Animal viruses included in the family are chicken anemia virus (CAV); beak and feather disease virus (BFDV); porcine circovirus (PCV); and pigeon circovirus. PCV was originally isolated from porcine kidney cell cultures. PCV replicates in the cell nucleus and produces large intranuclear inclusion bodies. See Murphy et al. (1999, Circoviridae p. 357-361, Veterinary Virology, 3rd ed. Academic Press, San Diego). There are currently two recognized types of PCV, PCV type 1 (PCV1) and PCV type 2 (PCV2). PCV1, isolated as a persistent contaminant of the continuous porcine kidney cell line PK-15 (ATCC CCL31), does not cause detectable cytopathic effects in cell culture and fails to produce clinical disease in pigs after experimental infection (see Allan G. (1995) Vet. Microbiol 44: 49-64; Tischer et al. (1982) Nature 295:64-66; and Tischer et al. (1986) Arch. Virol 91:271-276).
It is only very recently that some authors have thought that strains of PCV could be pathogenic and associated with the PMWS syndrome (Nayar et al. (1997) Can. Vet. J. 38: 385-387 and Clark E. G. (1997) Proc. Am. Assoc. Swine Prac. 499-501). Nayar et al. have detected PCV DNA in pigs having the PMWS syndrome using PCR techniques. PCV2, in contrast to PCV1, is closely associated with post weaning multisystemic wasting syndrome (PMWS) in weanling pigs (see Allan et al. (1998) Eur. J. Vet. Diagn. Investig. 10:3-10; Ellis et al. (1998) Can. Vet. J. 39:44-51 and Morozov et al. (1998), J. Clin. Microbiol. 36:2535-2541).
The nucleotide sequences for PCV1 are disclosed in Mankertz et al. (1997) J. Virol. 71:2562-2566) and Meehan et al. (1997) J. Gen. Virol. 78:221-227) and the nucleotide sequences for PCV2 are disclosed in Hamel et al. (1998) J. Virol. 72:5262-5267; Mankertz et al. (2000) Virus Res. 66:65-77 and Meehan et al. (1998) J. Gen. Virol. 79:2171-2179. Strains of PCV2 are disclosed in WO 00/01409 and have been deposited at the European Collection of Cell Cultures, Centre for Applied Microbiology & Research, Porton Down, Salisbury, Wiltshire SP4 OJG, United Kingdom and include: Accession No. V97100219; Accession No. V9700218; Accession No. V97100217; Accession No. V98011608; and Accession No. V98011609. WO 00/77216 also discloses PCV2.
As many as thirteen open reading frames (ORFs) have been identified in the PCV2 genome. ORF1 (Meehan et al., (1998)), alternatively designated as ORF4, comprises the nucleotides 398-1342 (GenBank Accession No. AF055392) and has the potential to encode a protein with a predicted molecular weight of 37.7 kD. ORF2 (Meehan et al., (1998); alternatively designated as ORF13, comprises the nucleotides 1381-1768 joined to 1-314 (GenBank Accession No. AF055392) and may encode a protein with a predicted molecular weight of 27.8 kD. Further description of the PCV2 ORFs 1-13 may be found in U.S. Pat. Nos. 6,368,601, 6,391,314, 6,660,272, 6,217,883, 6,517,843, 6,497,883 as well as AU 764379, EP 1019510, MX 221562, MX 216996, RU 2237492 and NZ 505008 which are incorporated herein by reference in the entirety.
ORF1 of PCV2 is highly similar (86% identity) to the ORF1 of the PCV1 isolate (Meehan et al., (1998)). The ORF1 protein of PCV1 has been partially characterized (Meehan et al., 1997; Mankertz et al., (1998) Virus genes 16:267-276) and has been shown to be essential for virus replication, most probably involved in the viral DNA replication.
Protein sequence identity between the respective ORF2s was lower (66% identity) than that between the ORF1s, but each of the ORF2s shared a highly conserved basic N-terminal region, similar to that observed in the N-terminal region of the major structural protein of the avian circovirus chicken anemia virus (CAV) (Meehan et al., 1998). Recently, Mankertz et al., in (1998) J. Gen. Virol. 79:381-384 has suggested that the ORF2 of the PCV1 isolate (designated ORF1 in Mankertz et al., 1998) codes for a capsid protein. The transcription analysis of the genome of PCV2 has not been published yet. Recent data obtained with the PCV1 isolate indicated that the ORF2 transcript is spliced (Mankertz et al., 1998).
Published studies to date on PCV2 used either tissue homogenate or cultured virus derived from field isolates. Tischer et al. ((1987) Arch Virol. 96:39-57) report that porcine kidney cells are stimulated to entry to the S phase in the cell cycle by D-glucosamine treatment. However, the treatment must be performed with caution because D-glucosamine is toxic for cell culture (see, Allan et al. (2000) J. Vet. Diagn. Investigation. 12:3-14).
There is a remaining need for methods for culturing circovirus including, for example, PCV1, PCV2 and other circoviruses, such that circovirus in high yield is possible. Such methods would be advantageous, in particular for preparation of PCV2 antigens as vaccines directed against PMWS. The present invention addresses that need. The invention relates to methods for growing and quantifying the infectious or antigenic amount and determining antibodies against circoviruses, in particular porcine circoviruses (PCV) that allow for in-process monitoring of the progress of the production of the virus in the batch culture.
Although porcine circovirus can be detected as a contaminating agent in pig tissue cultures, successful large-scale batch cultures of the virus require rapid assays to allow continual monitoring of the progress of viral production to obtain optimal yields. The object of the present invention, therefore, was to develop a method for monitoring the progress of the cultivation of a circovirus such as a porcine circovirus in vitro to be able to examine the ORFs kinetic expression. It was also intended to increase virus yield of a cell culture for the production of a vaccine that may require inactivated PCV or an avirulent PCV strain (e.g. through selection of an avirulent PCV strain after adaptation to various cell cultures and/or after treatment of infected cell cultures with mutagens or after genetic modification of the PCV) as live vaccine. In addition, the antigenic material obtained from grown porcine circoviruses can also be employed for diagnostic purposes. There is a need, therefore, to be able to periodically and rapidly monitor the progress of a batch cell culture of a circovirus under conditions that provide viral particles suitable for vaccine or other purposes. There is a need for monitoring methods that can give rapid results, rather than the labor-intensive and time-consuming methods currently employed for that purpose.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.